These days, in the automobile industry, the production of FF cars is increasing from the viewpoint of assuring weight reduction and passenger space. The production of 4WD cars is also increasing from the viewpoint of their functionality. Since these FF cars and 4WD cars transmit power and steering through front wheels, smooth power transmission is required even, for example, in the state that the front wheels are steered to the full lock position. Thus, the constant-velocity joint is indispensable as a component that transmits rotational motion of one axis to another axis at a constant velocity even when a crossing angle of the two axes which cross each other changes widely.
Among these constant-velocity joints, a ball-type constant-velocity joint has a structure in which a ball transmits torque as a rolling element. Flaking may take place in this structure because, during rotation, repetitive stress is applied to the ball and to the metal surface which the ball contacts due to a complex rolling and sliding motion under a high contact pressure.
In addition, higher output of recent engines has made the conditions of constant-velocity joint severer, and occurrence of flaking on the rolling surface of an outer race and an inner race and on the ball tends to be further accelerated. Therefore, the improvement of anti-flaking property is demanded.
As a grease composition for constant-velocity joints to overcome the above-described problems, a grease composition for constant-velocity joint containing a base oil, a urea thickener, sulfurized molybdenum dialkyldithiocarbamate, molybdenum disulfide, dialkyldithiophosphate, an oiliness agent comprising one or more vegetable oils is known (see, for example, Patent Document 1). However, these conventional grease compositions do not necessarily have enough anti-flaking property.
A grease composition for constant-velocity joints containing a base oil, a diurea thickener, a sulfurized molybdenum dialkyldithiocarbamate, a dialkyldithiophosphate, metal-free sulfur-phosphorus extreme pressure additive is also known (see, for example Patent Document 2). Although this grease composition exhibits an excellent anti-flaking property, the grease is not desirable in aspects of protecting global environment and of safety because it contains lead, which is heavy metal.    Patent Document 1 JP 2915611 B    Patent Document 1 JP 2989311 B